The invention relates to a deep drawing die having a top die and a bottom die, wherein the top die has a hold-down clamp and the bottom die has a die plate. The invention further relates to a method for deep drawing a workpiece using a deep drawing die.
Deep drawing is tensile-compressive forming of a plate or sheet metal to form a hollow body, or a pre-drawn hollow body is deformed to become such having a smaller cross-section. Both usually occur without any intended change to the sheet metal thickness. Round, oval and also angular cross-sections of components can be realized by use of deep drawing. The scope of deep-drawn parts ranges from small components having large drawing depth, e.g. beverage cans, to large-area components having different drawing depths, e.g. body parts of a motor vehicle.
In general, a deep drawing die comprises a top die having a punch and a hold-down clamp, as well as a bottom die having a die plate. A flat sheet metal plate is first positioned on the die plate, whereupon the top die moves in the direction of the bottom die. Once the top die contacts the sheet metal plate, a holding force is exerted on the plate by the hold-down clamp, where the drawing punch moves from the top die into the bottom die or into the die plate further downwardly and forms the plate into a cup-shaped deep-drawn part, for example.
The hold-down clamp serves to subject the sheet metal plate to a holding force during the forming process, such that wrinkling of the sheet metal due to tensile-compressive stresses in the sheet metal plate is prevented in the region between the hold-down clamp and the die plate. However, the holding force is selected such that the flow of material in the direction of the punch is not prevented.
The force for forming the sheet metal plate is transmitted from the punch to the base of the part to be deep-drawn into the flange of the sheet metal plate located between the draw ring and the hold-down clamp. The material of the sheet metal plate there flows in over the edge of the die plate or the draw ring from the edge of the plate or the flange, whereby the outer circumference is reduced.
The deep drawing process and the forming process are completed when the punch has reached its defined position within the die plate. Thereafter, the punch and the hold-down clamp or the top die return to their original positions. The base of the formed sheet metal plate has the original sheet thickness, where the cup wall has been stretched and the flange has been compressed.
In summary, the forming process takes place under the action of radial tensile and tangential compression stress, where the compression stress is caused by the excess material which would bring the flange to buckle if no hold-down clamp were provided.
Due to increasingly shorter product cycles and due to the increasing complexity of components and of assemblies in terms of shape, the aim is largely to eliminate in advance any process fluctuations in the complex process of deep drawing, so that rejects of parts in series operation is kept low and long periods in the start-up of production can be avoided.
Furthermore, for reasons of costs, it is the aim from an energy perspective to form thin-walled sheet metal plates.
Despite numerical simulation of forming processes, problems arise during the real implementation which generally result in an improvement of the tool or the die plate and are therefore costly. The reject rate of deep-drawn components also increases due to the increasing complexity of the components and the shorter product cycles.
It happens, for example, in particular with die plates having complex shapes, that forming stresses, arising when deep drawing the sheet metal plate, are so great that the material of the sheet metal plate is thinned by the forming process such that cracks or tears occur.
The requirements in terms of deep drawing complex components from thin-walled sheet metal plates seem to lie in opposite directions.